Zoom lens system

ABSTRACT

A zoom lens system includes a positive lens unit and a negative lens unit. Focusing in a focal length range from a shortest focal length condition to a middle focal length condition is performed by moving the positive lens unit, and focusing in a focal length range from the middle focal length condition to a longest focal length condition is performed by moving the negative lens unit. The zoom lens system fulfills the following condition: ##EQU1## where B f  is a distance from a rear end surface of the zoom lens system to a film at the shortest focal length condition and Δt is a diagonal length of the film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system, and more particularly, to a zoom lens system with an improved focusing method.

2. Description of the Prior Art

In a typical conventional zoom lens system, the same lens unit is used to perform focusing in the entire focal length range of zooming. However, usually, different lens units are the most suitable for moving for focusing in different focal length ranges. Therefore, to enable a closer photography while maintaining more excellent close photography performance, it is desirable to use, for focusing, different lens units in different focal length ranges.

In view of this, U.S. Pat. No. 5,042,926 discloses a three unit zoom lens system of negative, positive, negative configuration where the second lens unit is used to perform focusing in a focal length range (30 mm to 36 mm) from the shortest to the middle focal length conditions and the first lens unit is used to perform focusing in a focal length range (36 mm to 105 mm) from the middle to the longest focal length conditions. Also, a two-unit zoom lens system of positive, negative configuration is known as a product where all the lens units are moved out to perform focusing in the focal length range from the shortest to the middle focal length conditions and the first lens unit is moved out to perform focusing in the focal length range from the middle to the longest focal length conditions.

However, the former lens system presents a problem that the back focal length is extremely short in the focal length range from the shortest to the middle focal length conditions. If the back focal length is too short, the lens diameter increases, so that the size of the lens system increases. In the latter lens system where all the lens units are moved out to perform focusing, since the weight of the moved lenses is great, the size of the lens system increases due to a restriction on the mechanism for moving the lens units.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a zoom lens system enabling a closer photography while maintaining excellent optical performance and compactness.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which:

FIG. 1 is a cross-sectional view showing the lens arrangement of a first embodiment of the present invention;

FIG. 2 is a cross-sectional view showing the lens arrangement of a second embodiment of the present invention;

FIG. 3 is a cross-sectional view showing the lens arrangement of a third embodiment of the present invention;

FIG. 4 is a cross-sectional view showing the lens arrangement of a fourth embodiment of the present invention;

FIGS. 5A to 5I show aberration curves of the first embodiment of the present invention when the subject is at infinity;

FIGS. 6A to 6I show aberration curves of the first embodiment of the present invention when the subject distance is minimum;

FIGS. 7A to 7I show aberration curves of the second embodiment of the present invention when the subject is at infinity;

FIGS. 8A to 8I show aberration curves of the second embodiment of the present invention when the subject distance is minimum;

FIGS. 9A to 9I show aberration curves of the third embodiment of the present invention when the subject is at infinity;

FIGS. 10A to 10I show aberration curves of the third embodiment of the present invention when the subject distance is minimum;

FIGS. 11A to 11I show aberration curves of the fourth embodiment of the present invention when the subject is at infinity; and

FIGS. 12A to 12I show aberration curves of the fourth embodiment of the present invention when the subject distance is minimum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a zoom lens system of the present invention, a lens unit of positive refractive power is used to perform focusing in the focal length range from the shortest to the middle focal length conditions and a lens unit of negative refractive power is used to perform focusing in the focal length range from the middle to the longest focal length conditions. Further, the following condition (1) is fulfilled: ##EQU2## where B_(f) is a distance from the rear end surface of the lens system to the film at the shortest focal length condition and Δt is a diagonal length of the film.

In the focal length range from the shortest to the middle focal length conditions, by using the positive lens unit to perform focusing, the reduction in image illumination and the increase in coma can be restrained in close photography. In the focal length range from the middle to the longest focal length conditions, by using the negative lens unit to perform focusing, the increase in spherical aberration and coma can be restrained in close photography.

By fulfilling the condition (1), a sufficient back focal length can be secured in the focal length range from the shortest to the longest focal length conditions. When the lower limit of the condition (1) is exceeded, the rear surface of the lens system is too close to the film, so that the lens diameter increases. This is adverse to the size reduction along the diameter. Further, since the angle to the film surface is acuter, the illumination decreases according to the cos⁴ θ rule. When the upper limit of the condition (1) is exceeded, the total length of the optical system increases.

In the present invention, it is preferable to arrange the positive lens unit on the object side of the negative lens unit for the following reason: With such an arrangement, a required close photography performance can be secured and the total length of the optical system can be reduced. Therefore, for example, a long focal length zoom lens system and a compact zoom lens system for a lens shutter camera where the back focal length can be short to some degree can be realized.

In the present invention, it is preferable that a zoom lens system comprises two lens units of positive, negative configuration, that the first lens unit is used to perform focusing in the focal length range from the shortest to the middle focal length conditions, and that the second lens unit is used to perform focusing in the focal length range from the middle to the longest focal length conditions. With the two unit arrangement of positive, negative configuration, a compact zoom lens system for a lens shutter camera can be realized. Regarding the performance at the time of focusing, in the focal length range from the shortest to the middle focal length conditions, the decrease in illumination in close photography can be restrained by using the first lens unit to perform focusing. In the focal length range from the middle to the longest focal length conditions, the increase in coma and axial chromatic aberration in close photography can be restrained by using the second lens unit to perform focusing.

Alternatively, it is preferable that a zoom lens system comprises four lens units of positive, negative, positive, negative configuration, that the first lens unit is used to perform focusing in the focal length range from the shortest to the middle focal length conditions, and that the fourth lens unit is used to perform focusing in the focal length range from the middle to the longest focal length conditions. With the four unit arrangement of positive, negative, positive, negative configuration, a telephoto zoom lens system can be realized. Regarding the performance at the time of focusing, in the focal length range from the shortest to the middle focal length conditions, the increase in coma and field curvature can be restrained by using the first lens unit to perform focusing. In the focal length range from the middle to the longest focal length conditions, the increase in coma on the under side and lateral chromatic aberration can be restrained by using the fourth lens unit to perform focusing.

In the above-described four unit arrangement, to improve the performance at the time of focusing, the following conditions (2) and (3) are preferably fulfilled: ##EQU3## where f_(W) is a focal length of the entire lens system at the shortest focal length condition, f_(T) is a focal length of the entire lens system at the longest focal length condition, f₁ is a focal length of the first lens unit, and f₄ is a focal length of the fourth lens unit.

When the lower limit of the condition (2) is exceeded, it is difficult to correct coma and field curvature. When the upper limit of the condition (2) is exceeded, field curvature is generated on the over side. When the lower limit of the condition (3) is exceeded, it is difficult to restrain coma on the under side and lateral magnification. When the upper limit of the condition (3) is exceeded, coma is generated on the over side.

When a positive lens unit is arranged on the object side of a negative lens unit or when a two or a four unit arrangement is employed as described above, since the first lens unit is moved toward the object side in zooming from the shortest to the longest focal length conditions, the length of the lens barrel is maximum at the longest focal length condition. Performing focusing at the longest focal length condition without using the first lens unit prevents a further increase in the length of the lens barrel. This effect is greater when an independent actuator is used for moving each lens unit. When a piezoelectric actuator is used, each lens unit can independently be moved, so that focusing is easily performed.

In the present invention, it is preferable to arrange the negative lens unit on the object side of the positive lens unit for the following reason: With such an arrangement, a required close photography performance can be secured and a relatively long back focal length can be obtained, so that a zoom lens system can be realized with a zoom range starting from the shortest focal length condition.

It is preferable that a zoom lens system comprises two lens units of negative, positive configuration where the negative lens unit is arranged on the object side of the positive lens unit, that the second lens unit is used to perform focusing in the focal length range from the shortest to the middle focal length conditions, and that the first lens unit is used to perform focusing in the focal length condition from the middle to the longest focal length conditions. With the two unit arrangement of negative, positive configuration, a compact zoom lens system for a single-lens reflex camera with a zoom range starting from the shortest focal length condition can be realized. Regarding the performance at the time of focusing, in the focal length range from the shortest to the middle focal length conditions, the decrease in illumination is reduced by using the second lens unit to perform focusing. In the focal length range from the middle to the longest focal length conditions, the increase in coma can be restrained by using the first lens unit to perform focusing. This arrangement is effective in correcting distortion at the shorter focal length condition.

It is preferable that a zoom lens system comprises three lens units of negative, positive, negative configuration where the negative lens unit is arranged on the object side of the positive lens unit, that the second lens unit is used to perform focusing in the focal length range from the shortest to the middle focal length conditions, and that the first lens unit is used to perform focusing in the focal length range from the middle to the longest focal length conditions. With the three unit arrangement of negative, positive, negative configuration, a compact zoom lens system for a single-lens reflex camera with a zooming range starting from the shortest focal length condition can be realized. Regarding the performance at the time of focusing, in the focal length range from the shortest to the middle focal length conditions, the decrease in illumination can be reduced and the increase in coma and lateral chromatic aberration can be restrained by using the second lens unit to perform focusing. In the focal length range from the middle to the shortest focal length conditions, the increase in spherical aberration, coma and lateral chromatic aberration can be restrained by using the first lens unit to perform focusing. This arrangement is effective in correcting distortion at the longest focal length condition.

In the three unit arrangement of negative, positive, negative configuration, to improve the performance at the time of focusing, the following conditions (4) and (5) are preferably fulfilled: ##EQU4## where f₂ is a focal length of the second lens unit.

When the lower limit of the condition (4) is exceeded, it is difficult to restrain the increase in coma and lateral chromatic aberration in the focal length range from the shortest to the middle focal length conditions. When the upper limit of the condition (4) is exceeded, it is difficult to restrain the increase in field curvature on the under side in the focal length range from the middle to the longest focal length conditions. When the lower limit of the condition (5) is exceeded, it is difficult to restrain the increase in spherical aberration, coma and lateral chromatic aberration in the focal length range from the middle to the longest focal length conditions. When the upper limit of the condition (5) is exceeded, spherical aberration inclines toward the under side in the focal length range from the middle to the longest focal length conditions.

In the present invention, it is preferable to perform focusing by moving both of the positive and negative lens units used for focusing in each focal length range. By moving both of the lens units in each focal length range, illumination and aberrations such as coma, spherical aberration and chromatic aberration can more excellently be corrected.

Numerical data of first to fourth embodiments of the present invention are shown in Tables 1 to 4. In each table, ri (i=1,2,3, . . . ) represents a radius of curvature of an ith lens surface counted from the object side, di (i=1,2,3, . . . ) represents an ith axial distance counted from the object side, Ni (i=1,2,3, . . . ) and vi (i=1,2,3, . . . ) represent a refractive index and an Abbe number to the d-line of an ith lens surface counted from the object side. Focal lengths f of the entire lens system at the shortest focal length condition (W), at the middle focal length condition (M) and at the longest focal length condition (T), and an F-number FNO. are also shown.

In each table, the surfaces marked with asterisks (*) are aspherical and defined by the following expression representing the surface configuration of an aspherical surface: ##EQU5## where X is an amount of displacement from a reference surface along the optical axis, Y is a height in a direction vertical to the optical axis, C is a paraxial radius of curvature, ε is a conic constant, and Ai is an ith-order aspherical coefficient.

FIGS. 1 to 4 are cross-sectional views showing the lens arrangements of the first to fourth embodiments at the shortest focal length condition (W). Loci m1 to m4 in the figures schematically show the movements of a first lens unit Gr1, a second lens unit Gr2, a third lens unit Gr3 and a fourth lens unit Gr4 from the shortest focal length condition (W) to the longest focal length condition (T) during zooming.

The first embodiment comprises from the object side: a first lens unit Gr1 of positive refractive power including a negative meniscus lens element concave to the image side (first lens element G1) and a positive bi-convex lens element (second lens element G2); and a second lens unit Gr2 of negative refractive power including a positive meniscus lens element convex to the image side (third lens element G3) and a negative meniscus lens element concave to the object side (fourth lens element G4). The both side surfaces of the first lens element G1 and the both side surfaces of the third lens element G3 are aspherical. In the focal length range from the shortest focal length condition (W) to the middle focal length condition (M), the first lens unit Gr1 of positive refractive power is used to perform focusing as shown by an arrow F_(WM). In the focal length range from the middle focal length condition (M) to the longest focal length condition (T), the second lens element Gr2 is used to perform focusing as shown in an arrow F_(MT).

The second embodiment comprises from the object side: a first lens unit Gr1 of positive refractive power including a negative meniscus lens element concave to the image side (first lens element G1), a positive bi-convex lens element (second lens element G2) and a positive meniscus lens element convex to the object side (third lens element G3); a second lens unit Gr2 of negative refractive power including a negative bi-concave lens element (fourth lens element G4) and a positive bi-convex lens element (fifth lens element G5); a third lens unit Gr3 of positive refractive power including a negative bi-concave lens element (sixth lens element G6) and three positive bi-convex lens elements (seventh to ninth lens elements G7 to G9); and a fourth lens unit Gr4 of negative refractive power including a negative meniscus lens element concave to the image side (tenth lens element G10), a positive meniscus lens element convex to the image side (eleventh lens element G11) and a negative bi-concave lens element (twelfth lens element G12). In the focal length range from the shortest focal length condition (W) to the middle focal length condition (M), the first lens unit Gr1 of positive refractive power is used to perform focusing as shown by the arrow F_(WM). In the focal length range from the middle focal length condition (M) to the longest focal length condition (T), the fourth lens unit Gr4 of negative refractive power is used to perform focusing as shown by the arrow F_(MT).

The third embodiment comprises from the object side: a first lens unit Gr1 of negative refractive power including two negative meniscus lens elements concave to he image side (first and second lens elements G1 and G2) and a positive meniscus lens element convex to the object side (third lens element G3); and a second lens unit Gr2 of positive refractive power including a positive bi-convex lens element (fourth lens element G4), a positive meniscus lens element convex to the object side (fifth lens element G5), a negative bi-concave lens element (sixth lens element G6) and a positive bi-convex lens element (seventh lens element G7). The image side surface of the third lens element G3 is aspherical. In the focal length range from the shortest focal length condition (W) to the middle focal length condition (M), the second lens unit Gr2 of positive refractive power is used to perform focusing as shown by the arrow F_(WM). In the focal length range from the middle focal length condition (M) to the longest focal length condition (T), the first lens unit Gr1 of negative refractive power is used to perform focusing as shown by the arrow F_(MT).

The fourth embodiment comprises from the object side: a first lens unit Gr1 of negative refractive power including a negative bi-concave lens element (first lens G1) and a positive meniscus lens element convex to the object side (second lens G2); a second lens unit Gr2 of positive refractive power including a positive bi-convex lens element (third lens element G3) and a negative meniscus lens element concave to the object side (fourth lens element G4); and a third lens unit Gr3 of negative refractive power including a positive meniscus lens element convex to the image side (fifth lens element G5) and a negative meniscus lens element concave to the object side (sixth lens element G6). The both side surfaces of the fourth lens element G4 and the object side surface of the fifth lens element G5 are aspherical. In the focal length range from the shortest focal length condition (W) to the middle focal length condition (M), the second lens unit Gr2 of positive refractive power is used to perform focusing as shown by the arrow F_(WM). In the focal length range from the middle focal length condition (M) to the longest focal length condition (T), the first lens unit Gr1 of negative refractive power is used to perform focusing as shown by the arrow F_(MT).

FIGS. 5A to 5I through 12A to 12I show aberration curves of the first to fourth embodiments. FIGS. 5A to 5I, 7A to 7I, 9A to 9I and 11A to 11I show aberration curves when the subject is at infinity. FIGS. 6A to 6I, 8A to 8I, 10A to 10I and 12A to 12I show aberration curves when the subject distance is minimum. The object distance D is 0.8 m in the first embodiment (FIGS. 6A to 6I), 1.5 m in the second embodiment (FIGS. 8A to 8I), 0.8 m in the third embodiment (FIGS. 10A to 10I) and 0.6 m in the fourth embodiment (FIGS. 12A to 12I). FIGS. 5A to 5C, 6A to 6C, 7A to 7C, 8A to 8C, 9A to 9C, 10A to 10C, 11A to 11C and 12A to 12C show aberration curves at the shortest focal length condition (W). FIGS. 5D to 5F, 6D to 6F, 7D to 7F, 8D to 8F, 9D to 9F, 10D to 10F, 11D to 11F and 12D to 12F show aberration curves at the middle focal length condition (M). FIGS. 5G to 5I, 6G to 6I, 7G to 7I, 8G to 8I, 9G to 9I, 10G to 10I, 11G to 11I and 12G to 12I show aberration curves at the longest focal length condition (T). The solid lines d represent aberrations to the d-line. The dash and dotted lines g represent aberration curves to the g-line. The chain double-dashed lines c represent aberration curves to the c-line. The broken lines SC represent sine conditions. The broken lines DM and the solid lines DS represent astigmatism on the meridional and sagittal image planes, respectively.

Values of the conditions (1) to (5) in the first to fourth embodiments are shown in Table 5. The aberration coefficients of the second to fourth embodiments at the shortest focal length condition (W) and the longest focal length condition (T) when the subject is at infinity and the subject distance is minimum are shown in Tables 6 to 8. As the aberration coefficients when the subject distance is minimum, aberration coefficients when focusing is performed by using each of the lens unit used for focusing (the first lens unit Gr1, the second lens unit Gr2 and the fourth lens unit Gr4) are also shown to compare the case where the lens unit actually used for focusing in the focal length range is used and the case where the other of the lens units used for focusing is used instead.

It is understood from Tables 6 to 8 that it is advantageous in correcting aberrations to use different lens units for focusing in different focal length ranges in each embodiment. The closer to 0 the aberration coefficient is, the more excellent the aberration performance is. Therefore, since the aberration coefficient is closer to 0 when the first lens unit Gr1 is used to perform focusing than when the fourth lens unit Gr4 is used to perform focusing at the shortest focal length condition (W) and closer to 0 when the fourth lens unit Gr4 is used to perform focusing than when the first lens unit Gr1 is used to perform focusing at the longest focal length condition (T), field curvature is more excellently corrected by the previously-described focusing method of the second embodiment.

In the second embodiment, at the shortest focal length condition (W), coma and field curvature are excellently corrected, and at the longest focal length condition (T), although spherical aberration and coma are not excellent, the aberration level is excellent since they are corrected on higher order, and field curvature is excellently corrected. In the third embodiment, while the aberration coefficient is substantially the same at the shortest focal length condition (W), coma is excellently corrected at the longest focal length condition (T). In the fourth embodiment, while the aberration coefficient is the same at the shortest focal length condition (W), spherical aberration is excellently corrected at the longest focal length condition.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.

                  TABLE 1     ______________________________________     (1st Embodiment)     f = 39.1 to 55.0 to 102.1   FNO. = 3.6 to 5.1 to 9.4     Radius of Axial       Refractive  Abbe     Curvature Distance    Index       Number     ______________________________________     r1* 34.261                   d1      2.500 N1   1.84506                                             ν1                                                  23.66     r2* 21.818                   d2      4.400     r3  128.904                   d3      4.900 N2   1.58913                                             ν2                                                  61.25     r4  -14.590             d4   16.559 to 10.810 to 4.275     r5* -67.105                   d5      3.300 N3   1.58340                                             ν3                                                  30.23     r6* -34.388                   d6      4.880     r7  -11.956                   d7      1.000 N4   1.78100                                             ν4                                                  44.55     r8  -44.532     Σd = 37.539 to 31.789 to 25.254     Aspherical Coefficients     r1    :ε = 1.0000                           r2    :ε = 1.0000           A4 = -0.16966 × 10.sup.-3                                 A4 = -0.13003 × 10.sup.-3           A6 = -0.79936 × 10.sup.-6                                 A6 = -0.85242 × 10.sup.-6           A8 = 0.29138 × 10.sup.-8                                 A8 = 0.27253 × 10.sup.-7           A10 = 0.77000 × 10.sup.-10                                 A10 = -0.21163 × 10.sup.-9           A12 = -0.14876 × 10.sup.-11                                 A12 = -0.13969 × 10.sup.-11                                 A14 = -0.65974 × 10.sup.-18                                 A16 = -0.10790 × 10.sup.-19     r5    :ε = 1.0000                           r6    :ε = 1.0000           A3 = -0.37891 × 10.sup.-3                                 A3 = -0.36195 × 10.sup.-3           A4 = 0.24109 × 10.sup.-3                                 A4 = 0.13866 × 10.sup.-3           A5 = -0.41114 × 10.sup.-4                                 A5 = -0.12138 × 10.sup.-4           A6 = 0.45938 × 10.sup.-5                                 A6 = -0.29489 × 10.sup.-6           A7 = 0.87306 × 10.sup.-7                                 A7 = 0.39458 × 10.sup.-7           A8 = -0.44434 × 10.sup.-7                                 A8 = 0.44473 × 10.sup.-7           A9 = -0.42186 × 10.sup.-9                                 A9 = -0.64254 × 10.sup.-8           A10 = 0.34225 × 10.sup.-9                                 A10 = 0.10949 × 10.sup.-9           A11 = 0.82545 × 10.sup.-11                                 A11 = 0.25726 × 10.sup.-10           A12 = -0.18935 × 10.sup.-11                                 A12 = -0.17438 × 10.sup.-11           A13 = 0.74972 × 10.sup.-15                                 A13 = 0.68692 × 10.sup.-13           A14 = 0.58928 × 10.sup.-15           A15 = 0.90229 × 10.sup.-16           A16 = 0.10125 × 10.sup.-16     ______________________________________

                  TABLE 2     ______________________________________     (2nd Embodiment)     f = 102.5 to 300.0 to 390.0   FNO. = 4.6 to 5.8 to 6.9     Radius of Axial       Refractive  Abbe     Curvature Distance    Index       Number     ______________________________________     r1  130.023                   d1      2.504 N1   1.85000                                             ν1                                                  40.04     r2  75.029                   d2      0.100     r3  75.029                   d3      8.008 N2   1.49310                                             ν2                                                  83.58     r4  -224.260                   d4      0.200     r5  73.713                   d5      4.597 N3   1.48749                                             ν3                                                  70.44     r6  139.197             d6    4.000 to 65.500 to 82.207     r7  -94.599                   d7      1.401 N4   1.74400                                             ν4                                                  44.93     r8  44.699                   d8      4.361     r9  51.142                   d9      2.399 N5   1.75520                                             ν5                                                  27.51     r10 -1371.384             d10  52.632 to 12.659 to 3.801     r11 -733.945                   d11     1.498 N6   1.84666                                             ν6                                                  23.83     r12 43.963                   d12     2.700     r13 319.419                   d13     3.006 N7   1.59270                                             ν7                                                  35.45     r14 -80.974                   d14     1.408     r15 75.821                   d15     2.403 N8   1.51823                                             ν8                                                  58.96     r16 -226.290                   d16     0.100     r17 46.868                   d17     4.497 N9   1.48749                                             ν9                                                  70.44     r18 -56.861             d18  30.375 to 8.848 to 0.999     r19 196.053                   d19     1.194 N10  1.75450                                             ν10                                                  51.57     r20 33.838                   d20     3.309     r21 -222.323                   d21     3.205 N11  1.67339                                             ν11                                                  29.25     r22 -28.596                   d22     0.010 N12  1.51400                                             ν12                                                  42.83     r23 -28.596                   d23     1.203 N13  1.69680                                             ν13                                                  56.47     r24 422.978     Σd = 135.109 to 135.109 to 135.109     ______________________________________

                  TABLE 3     ______________________________________     (3rd Embodiment)     f = 29.2 to 41.0 to 76.5   FNO. = 4.1 to 4.8 to 5.8     Radius of Axial       Refractive  Abbe     Curvature Distance    Index       Number     ______________________________________     r1  46.392                   d1      1.500 N1   1.77551                                             ν1                                                  37.90     r2  22.920                   d2      4.400     r3  46.082                   d3      1.450 N2   1.71300                                             ν2                                                  53.93     r4  19.322                   d4      3.800     r5  25.007                   d5      5.600 N3   1.67339                                             ν3                                                  29.25     r6  80.909                   d6      0.050 N4   1.51790                                             ν4                                                  52.31     r7* 54.506             d7   41.232 to 23.855 to 3.900     r8  24.891                   d8      4.050 N5   1.58913                                             ν5                                                  61.11     r9  -85.435                   d9      0.160     r10 19.125                   d10     3.450 N6   1.51680                                             ν6                                                  64.20     r11 122.331                   d11     1.820     r12 -59.023                   d12     4.550 N7   1.80741                                             ν7                                                  31.59     r13 17.358                   d13     2.450     r14 4379.433                   d14     2.450 N8   1.59551                                             ν8                                                  39.22     r15 -24.988     Σd = 76.962 to 59.584 to 39.630     Aspherical Coefficients     r7    :ε = 1.0000           A4 = -0.99880 × 10.sup.-5           A6 = 0.61836 × 10.sup.-8           A8 = -0.20792 × 10.sup.-9           A10 = 0.73220 × 10.sup.-12           A12 = -0.13569 × 10.sup.-14     ______________________________________

                  TABLE 4     ______________________________________     (4th Embodiment)     f = 28.7 to 50.0 to 77.5   FNO. = 3.6 to 6.0 to 8.1     Radius of Axial       Refractive  Abbe     Curvature Distance    Index       Number     ______________________________________     r1  -162.844                   d1      1.000 N1   1.80100                                             ν1                                                  46.54     r2  18.910                   d2      1.131     r3  21.538                   d3      5.000 N2   1.80750                                             ν2                                                  35.43     r4  85.222             d4   19.040 to 13.941 to 4.052     r5  8.644                   d5      5.000 N3   1.48749                                             ν3                                                  70.44     r6  -26.098                   d6      0.627     r7* -12.255                   d7      2.800 N4   1.75690                                             ν4                                                  31.79     r8* -23.577             d8    9.725 to 3.962 to 3.600     r9* -19.973                   d9      3.350 N5   1.58340                                             ν5                                                  30.23     r10 -15.799                   d10     4.600     r11 -10.000                   d11     1.000 N6   1.61800                                             ν6                                                  63.39     r12 -50.713     Σd = 53.274 to 42.412 to 32.160     Aspherical Coefficients     r7    :ε = 1.0000                           r8    :ε = 1.0000           A4 = 0.41138 × 10.sup.-3                                 A4 = 0.51772 × 10.sup.-3           A6 = 0.16208 × 10.sup.-5                                 A6 = 0.41869 × 10.sup.-5           A8 = -0.35351 × 10.sup.-7                                 A8 = 0.30548 × 10.sup.-7           A10 = -0.32191 × 10.sup.-9                                 A10 = 0.14342 × 10.sup.-9     r9    :ε = 1.0000           A3 = 0.55964 × 10.sup.-4           A4 = 0.57015 × 10.sup.-4           A5 = -0.16593 × 10.sup.-5           A6 = -0.31963 × 10.sup.-6           A7 = -0.34509 × 10.sup.-7           A8 = 0.40595 × 10.sup.-7           A9 = -0.21819 × 10.sup.-8           A10 = -0.99837 × 10.sup.-10           A11 = -0.15082 × 10.sup.-11           A12 = 0.79424 × 10.sup.-12     ______________________________________

                  TABLE 5     ______________________________________                1st                embodi- 2nd       3rd     4th     Conditions ment    embodiment                                  embodiment                                          embodiment     ______________________________________      ##STR1##  0.203   0.912     0.956    0.219      ##STR2##  --      0.73      --      --      ##STR3##  --      9.48      --      --      ##STR4##  --      --        --      1.37      ##STR5##  --      --        --      1.40     ______________________________________

                  TABLE 6     ______________________________________                      2nd embodiment                      (positive, negative                      positive, negative)                      W     T     ______________________________________     Spherical ∞      0.81    6.26     aberration               Gr1          0.82    8.2               Gr4          0.84    9.9     Coma      ∞      -0.60   -0.89               Gr1          -0.47   0.69               Gr4          -0.89   -1.74     Field     ∞      0.02    0.14     curvature Gr1          -0.11   -0.58               Gr4          0.38    0.14     ______________________________________

                  TABLE 7     ______________________________________                      3rd embodiment                      (negative, positive)                      W     T     ______________________________________     Spherical ∞      3.57    8.01     aberration               Gr1          3.68    12.8               Gr2          4.24    12.3     Coma      ∞      0.57    -0.32               Gr1          0.56    -0.07               Gr2          0.54    0.38     Field     ∞      0.18    -0.05     curvature Gr1          0.17    -0.04               Gr2          0.15    -0.01     ______________________________________

                  TABLE 8     ______________________________________                      4th embodiment                      (negative, positive,                      negative)                      W     T     ______________________________________     Spherical ∞      0.83    -34.5     aberration               Gr1          1.03    -23.8               Gr2          1.00    -38.3     Coma      ∞      0.16    -0.71               Gr1          0.16    -0.165               Gr2          0.13    -1.53     Field     ∞      0.13    0.01     curvature Gr1          0.14    0.04               Gr2          0.16    -0.01     ______________________________________ 

What is claimed is:
 1. A zoom lens system comprising:a positive lens unit; and a negative lens unit, wherein focusing in a focal length range from a shortest focal length condition to a middle focal length condition is performed by moving the positive lens unit, and wherein focusing in a focal length range from the middle focal length condition to a longest focal length condition is performed by moving the negative lens unit, and wherein the following condition is fulfilled: ##EQU6## where B_(f) is a distance from a rear end surface of the zoom lens system to a film at the shortest focal length condition and Δt is a diagonal length of the film.
 2. A zoom lens system as claimed in claim 1, wherein said negative lens unit is arranged on an object side of the positive lens unit.
 3. A zoom lens system as claimed in claim 1, wherein zooming is performed by moving both of the positive and negative lens units.
 4. A zoom lens system as claimed in claim 1 wherein the negative lens unit includes a pair of negative meniscus lens elements concave to an image side and a positive meniscus lens element convex to an object side.
 5. A zoom lens system as claimed in claim 1 wherein the positive lens unit includes a positive meniscus lens element convex to an object side.
 6. A zoom lens system as claimed in claim 5 wherein the positive lens unit further includes two positive biconvex lens elements and a negative biconcave lens element.
 7. A zoom lens system as claimed in claim 1 wherein the focusing movement of the positive lens unit is toward an object side while the focusing movement of the negative lens unit is toward an image side.
 8. A zoom lens system consisting of from an object side:a first lens unit having a negative refractive power; and a second lens unit having a positive refractive power, wherein focusing in a focal length condition from a shortest focal length condition to a middle focal length condition is performed by moving the second lens unit, and wherein focusing in a focal length range from the middle focal length condition to a longest focal length condition is performed by moving the first lens unit.
 9. A zoom lens system as claimed in claim 8, wherein the following condition is fulfilled: ##EQU7## where B_(f) is a distance from a rear end surface of the zoom lens system to a film at the shortest focal length condition and Δt is a diagonal length of the film. 